The invention relates generally to a hydraulic system embodying a demand compensated principle in which the output of a variable displacement pump can be made proportional to the instantaneous load demand of one or more fluid motors powered by the pump and more particularly to a demand compensated hydraulic system which includes a servo motor control valve which controls and is closed by servo feedback from the motor and which further includes a dither to return the variable displacement pump to stand-by as the demand approaches zero.
In the past, demand compensated hydraulic systems with closed loop control of a motor tended to "hang up" at load pressure and not allow the pump to return to the standby condition when the load stopped moving. This was because the internal pressure drop across the servo motor control valve would cause the pressure at the motor to be insufficient to operate the motor to activate the servo mechanism to close the valve and thereby cause the pump to go into the stand-by condition but would be sufficient to feed back through the demand feedback system to prevent the pump from going to the stand-by condition. In this hung up condition, the pump would continue to pump fluid through the demand feedback system resulting in heat buildup in the fluid and power loss due to non-standby operation of the pump.